Flameproofing of cellulose

ABSTRACT

A process for flameproofing cellulosic materials, such as cotton and rayon. The cellulosic material is (a) oxidized to carboxycellulose, (b) the carboxycellulose is reacted with either a water-soluble alkali metal hydroxide, a buffer solution of water-soluble alkali metal salts or ammonia, and (c) the product of (b) is suspended in an aqueous solution of a water-soluble polyvalent cationic salt.

United States atent Gupta et al. [451 May 22, 1973 54] FLAMEPROOFING 0F CELLULOSE 2,495,767 1 1950 Reid et al. ..s 120 x [75] Inventors: Virendra N. Gupta; Renald L. $23 at Lafond both of Hawkesbury, Om

lafio, Canada OTHER PUBLICATIONS Asslgneei Canadian International Paper Little, Robert W., Flameproofing Textile Fabrics,

p y Montreal, Quebec, Cana 1947, pages 169, 170, 172

22 F1 d: A 23 1970 l l l e pr Primary ExaminerLeon D. Rosdol PP 31,374 Assistant Examiner-Harold Wolman Attorney-R. G. McClenahan and Eli J. McKhool 52 U.S. Cl. ..8 116 R 8 120,117 138, l l I I g, 57 ABSTRACT 3 D06"! (309k A process for flameproofing cellulosic materials, such Fleld of Search ..8/1 16, l as cotton and rayo The cellulosic material is (a) QX idized to carboxycellulose, (b) the carboxycellulose is References Clled reacted with either a water-soluble alkali metal hydroxide, a buffer solution of water-soluble alkali UNITED STATES PATENTS metal salts or ammonia, and (c) the product of (b) is 2,805,176 9/1957 Sindl ..8/12O UX suspended in an aqueous solution of a water-soluble 2,448,153 8/1948 Reid et al. ..8/12O polyvalent cationic salt. 2,6l7,707 ll/l952 Daul et al. ..8/120 2,420,949 5/1947 Hager et al. ..260/232 4 Claims, N0 Drawings FLAMEPROOFING F CELLULOSE This invention relates to the conversion of cellulose into a flameproof material.

A great disadvantage in the use of cellulosic materials, such as cotton, wood pulp, rayon, etc., in any end product, is its flammability. Because of this fire hazard, it is imperative that cellulosics be flameproofed for many uses.

In the literature on flameprooflng there are several terms, e.g., flameproofness, flame resistance, flame retardance, etc., which are difficult to define and to be distinguished clearly from one another. However, every material should fall in one of the following classes.

i. The material ignites when placed in a flame, and when removed from the flame continues to burn with a flame until completely burnt to ash.

ii. The material ignites when placed in a flame, but when removed from the flame stops burning with a flame but continues to glow (an afterglow).

iii. The material ignites or glows when placed in a flame, but when removed from the flame stops burning and leaves no afterglow (self-extinguishing).

iv. The material does not ignite even in the flame.

in view of the fact that any material derived from cellulose must contain both carbon and hydrogen, the chances of its falling in class (iv) are extremely small. The aim of research in this field is, therefore, to convert cellulose into a material falling in class (iii) which can be classified as safe for a very large number of end uses.

In the past flameproofness in cellulosic materials has been introduced by two methods. In the first, the cellulosic material is impregnated with a large amount of inorganic salts, like ammonium phosphate, borates etc. These salts may be soluble or insoluble in water. If they are soluble in water they have a great disadvantage of being leached out during washing, resulting in a loss of flameproofness. The water-insoluble salts, on the other hand, may not have this disadvantage. However, the retention of both water-soluble and insoluble salts by the fiber web is extremely poor.

The other method used to impart flameproofness is to chemically bind the flameproofing reagents to the cellulose molecules. These techniques have been more successful than the other method described above but are very expensive.

In the present invention it was conjectured that if an inorganic or metal ion were chemically attached to the cellulose molecule, it would impart flameproofness to cellulose. This would then avoid the problem of retaining salts in the fiber web. Also, this kind of flameproofing should be stable to washing.

Therefore, it is an object of the present invention to provide an inexpensive process whereby cellulosic materials are made flameproof and, in addition, stable to washing.

It is a further object of the present invention to provide a process whereby cellulosic materials will not only be made flameproof but also will have no afterglow, i.e., they will be self-extinguishing.

it has been found that a fibrous, water-insoluble cellulose carboxylate which is both flameproof and selfextinguishing is obtained when a cellulosic material is reacted in accordance with the following process steps:

a. oxidizing the cellulosic material to a carboxycellulose having a carboxyl content of from about 5 percent to about 25 percent;

b. reacting the carboxycellulose with a compound selected from the group consisting of water-soluble alkali metal hydroxides, buffer solutions of water-soluble alkali metal salts and ammonia;

c. suspending the product of (b) above in an aqueous solution containing a water-soluble, polyvalent, cationic salt selected from the group consisting of trivalent metal salts, divalent heavy metal salts and divalent alkaline earth metal salts.

A great variety of cellulosic-containing starting materials can be employed. Exemplary of such cellulosic materials are wood pulps, such as kraft or sulfite pulp, cotton, cotton linters and rayon. While it is preferred to employ a 6-carboxy cellulose obtained by oxidizing the cellulosic starting material with nitrogen dioxide, it is to be understood that the present application is not limited thereto. Carboxycelluloses wherein the carboxyl group is substituted at the C or C position can also be employed. It should also be understood that the present invention is not limited to treating the cellulosic starting material with N0 gas as the oxidizing agent. Other oxidizing agents can be used with equal effect. However, as a matter of convenience in describing the invention, the following detailed description of the invention will discuss the treatment of 6-carboxy cellulose, obtained by oxidizing the cellulosic starting material with nitrogen dioxide gas.

The cellulosic starting material is reacted with nitrogen dioxide under process conditions well known in the art. It is known that if the cellulosic starting material is fully reacted, a 6-carboxy cellulose having a carboxyl content of about 25 percent is obtained. This indicates that the 6-hydroxy group on the anhydroglucose monomeric unit in the cellulose chain has been completely converted to -carboxyl. Improved flameproofness is exhibited at a carboxyl content of from about 5 percent to about 25 percent. However, it is preferred, for the purposes of the present invention, that the reaction be carried to a point where from about 8 percent to about 12 percent carboxyl content is obtained.

The 6-carboxy cellulose is then suspended in a solution containing a compound selected from the group consisting of water-soluble alkali metal hydroxides, buffer solutions of water-soluble alkali metal salts and ammonia. The 6-carboxy cellulose is suspended in from about-0.6 to about 1.0 equivalents of an alkali metal hydroxide or ammonia, or an excess of buffer solution of water-soluble alkali metal salts and treatment is continQ ued until neutralization is effected. The resulting product is a fibrous, water-insoluble cellulosecarboxylate. Exemplary of the alkali metal hydroxides which can be employed are sodium hydroxide and potassium hydroxide. Exemplary of the buffer solutions of alkali metal salts which can be employed are sodium sulfite-sodiumbisulfite, potassium sulfite-potassium bisulfite, ammo nium sulfite-ammonium bisulfite, ammonium dihydro j gen phosphate-diammonium hydrogen phosphate, potassium monohydrogen phosphate-potassiumdihydrogen phosphate. In the present process it is preferred to. employ a buffer solution of sodium sulfite-sodium 'bi-.

sulfite. The reaction product is then filtered, washed with water and air-dried. The resulting water-insoluble 'cellulosecarboxylate which is in a highly swollen state, is then reacted with a water-soluble salt having a polyvalent cation. The cat-.

ion can be either a trivalent metal, a divalent heavy metal or a divalent alkaline earth metal. Exemplary of;

the trivalent metal cations are aluminum, ferric and chromic. Exemplary of the divalent heavy metal cations are zinc, ferrous, cupric and nickel. Exemplary of the divalent alkaline earth metal cations are calcium and magnesium.

Trivalent salts which can be used are aluminum sulfate, alum, aluminum chloride, ferric sulfate and potassium chromium sulfate. Divalent heavy metal salts which can be used are zinc sulfate, ferrous ammonium sulfate, copper sulfate and nickel acetate. Divalent alkaline earth metal salts which can be used are calcium chloride and magnesium sulfate.

The trivalent metal cations are preferred, with aluminum being especially preferred. An added advantage of aluminum, at least in certain instances, is that it imparts no color.

The highly swollen, water-insoluble carboxycellulose salt is stirred for about 15 minutes at room temperature with the water-soluble polyvalent containing salt and the product is filtered, washed with water and air-dried. The final product is a water-insoluble, fibrous material. The material has been deswelled and can now be formed into a sheet which is both flameproof and free of afterglow.

An important property in a flameproof material is the durability of flameproofness. From this point of view the durability of flameproofness can be divided into the following three categories:

i. Temporary or non-durable Flameproofness which is lost by one washing with water, or detergent solution and/or by dry cleaning.

ii. Semidurable Flameproofness which will withstand three to five launderings with detergent solutions and/or three to five dry cleanings.

iii. Durable This type of flameproofness will withstand extensive launderings and dry cleanings.

The durability of flameproofness of aluminum cellulose-carboxylate was tested against water, dry cleaning and laundering with detergents. As described hereinafter in Examples X and X! the flameproofness was not lost by any number of water washings or dry cleanings.

The conditions chosen for laundering were very similar to those which exist in a household washing machine. From the results described hereinafter in Example XII it is evident that the flameproofness of aluminum cellulosecarboxylate is stable for 4-5 launderings and after about launderings reaches that of sodium cellulosecarboxylate and hence may be termed semidurable against detergents. It must be noted, however, that the flame characteristics of sodium cellulosecarboxylate which is formed after launderings are much superior to those of untreated cellulose.

The flameproofness lost during laundering as discussed above can be regenerated easily by the treatment of sodium cellulosecarboxylate with alum.

The following examples illustrate the process of the present invention.

EXAMPLE I Five grams of -carboxycellulose, having a l2 percent carboxyl content, was prepared by oxidizing wood pulp with nitrogen dioxide. The carboxycellulose was suspended for 4 hours in a solution containing 5 grams of sodium bisulfite and grams of sodium sulfite dissolved in 500 mls. of water. The resulting cellulosic product was filtered on a Buchner funnel using a Whatman No. 4 filter paper, washed with 2 liters of water and was then air-dried.

EXAMPLE II TABLE 1 Color of Product Colorless Chars in flame, no

Aluminum Sulfate afterglow 2. Ferric Sulfate Brown 3. Potassium Chromium Green Sulfate 4. Zinc Sulfate Colorless Burns with a little flame, leaves afterflow 5 Ferrous Ammonium Brown Burns without flame,

Sulfate leaves afterglow 6 Copper Sulfate Blue Burns with flame,

afterglow 7. Nickel Acetate Aqua 8. Calcium Chloride Colorless 9. Magnesium Sulfate Colorless EXAMPLE III Ten grams of 6-carboxycellulose, having a 12 percent carboxyl content, was obtained by oxidizing wood pulp with nitrogen dioxide. The -carboxycellulose was suspended for 4 hours in a one-liter solution containing 30 grams of sodium sulfite and 10 grams of sodium bisulfite. The resulting product, sodium cellulosecarboxylate, was filtered on a Buchner funnel using a Whatman No. 4 filter paper, washed with 2 liters of water and airdried.

In each of the experiments listed below in Table 2, 10 grams of sodium cellulose carboxylate, prepared in the manner described above, was suspended for 15 minutes, with stirring, in a solution containing 10 grams of aluminum sulfate in one liter of water. The resulting product, aluminum cellulosecarboxylate, was filtered, washed with 2 liters of water to remove excess aluminum sulfate and then air-dried. In experiments 2 through 5 inclusive, in Table 2 below, the aluminum cellulosecarboxylate, prepared as described above, was slurried with an untreated bleached kraft pulp (Cellate-Canadian International Paper Co.) for 5 to 10 minutes. The percentages of Cellate and aluminum cellulosecarboxylate listed in Table 2 are weight percentages. Handsheets with basis weights of 60 grams per square meter were then prepared. These were dried at room temperature and maintained at 50 percent relative humidity.

The measurements listed in Table 2 were carried out using a Society of the Plastics Industry flammability tester (ASTM-Dl433). When using this tester the specimen is mounted at a 45 angle and the time necessary to burn 6 inches of the sheet is measured. From this measurement one can calculate the burning rate in inches per second.

TABLE 2 Experiment 1 2 3 4 5 6 Cellate by weight) 100 95 90 75 50 Al Cellulose carboxylate 10 25 50 100 by weight) Burning time for 6", secs 4.2 4.7 4.9 5.9 7.0 Burning rate, Inches/sec 1.43 1.28 1.22 1.02 0.86 Burning temp. "F 550 500 550 550 550 indicates that sample did not burn It is clear from the above results that the rate of buming decreases with increasing amounts of aluminum cellulose carboxylate. Visual examination of the burnt sheet showed a network of black particles, probably resulting from the presence of aluminum cellulosecarboxylate which had no afterglow. However, Cellate which was not flameproof, burnt with the flame propagating around the flameproof aluminum cellulosecarboxylate. It is, therefore, possible to use aluminum cellulosecarboxylate in a blend, for example, for textile use so as to decrease the rate of burning.

1n the preparation of blends it is not necessary to prepare aluminum cellulosecarboxylate beforehand. Handsheets could be prepared from blends of Cellate pulp and sodium cellulosecarboxylate and the handsheets could then be treated with a solution of aluminum sulfate, followed by removal of excess aluminum sulfate with water.

EXAMPLE IV TABLE 3 Carboxyl content Flame characteristics 1.3 Burns in flame and continues burning outside the flame.

6.0 Burns without flame, some afterglow 9.4 Glows in flame, no afterglow EXAMPLE V Five grams of -carboxycellulose (12.5 percent carboxyl) was treated with 0.2, 0.4, 0.6, 0.8, and 1.0 equivalents of sodium hydroxide in 250 mls. of water for 1 minute at room temperature. The sodium salt was quickly filtered, washed with water and treated with 200 mls. of a solution containing 4 grams of aluminum sulfate. The material was then filtered, washed with water and air-dried. The following results, tabulated in Table 4 below, were obtained.

TABLE 4 NaOH Remarks on Flame characteristics Equivalents Sodium salt of aluminum slat Nil Burns completely 0.2 Yield good, no slimy Glows in flame, a

touch, good filtration little afterglow a yellow color in filtrate Glows in flame, no

0.6 0.8 Yield poor, slimy touch,

slow filtration, some yellow color in filtrate EXAMPLE VI A sample of carboxycellulose containing 12.6 percent carboxyl was exposed to ammonia gas for 1 minute. Excess ammonia was removed from the sample by passing air for 5 minutes. The product was poured into water, filtered, washed with water and then treated with 4 grams of aluminum sulfate in 200 mls. H O. The product was then filtered, washed with water and airdried. The final product was slightly yellow, glowed in a flame but had no afterglow when taken out of the flame. In another experiment the ammonia and air treated carboxycellulose was directly treated with alum to give a brownish yellow product with selfextinguishing properties.

EXAMPLE VII Samples of sodium cellulosecarboxylate were prepared from carboxycellulose containing 12.6 percent carboxyl, in accordance with Example I, and were then treated with 0.2, 0.4, 0.6, 0.8 and 1.0 equivalents of aluminum sulfate as in Example IV. The flame properties of the products are given below in Table 5.

TABLE 5 Aluminum sulfate, equiv. 0

Flame Properties Burns in flame,

afterglow Glows in flame, a little afterglow EXAMPLE Vlll no disadvantage. 50

Flows in fame, no afterglow TABLE 6 Carboxyl content afterglow EXAMPLE IX Cotton was oxidized and treated like rayon, as described in Example V111. If the carboxyl content of cotton was at least 7-8 percent, a self-extinguishing aluminum salt was obtained.

Glowed in flame. N0

TABLE 7 Flame characteristics Burned in flame. Stopped burning outside the flame. Very little afierglow. l0.2 Flowed in flame. No afierglow.

Carboxyl content EXAMPLE X EXAMPLE XI The procedure reported in Example X was repeated with several organic solvents including methanol, acetone, benzene, carbon tetrachloride and petroleum ether. The flameproofness was not lost by any number of washings with any of these solvents.

EXAMPLE XII One hundred grams of aluminum cellulosecarboxylate prepared from carboxycellulose containing 12 percent carboxyl was suspended in a 2% liter solution containing 3.75 grams of Bold detergent at 50C. The slurry was shaken for 15 minutes and then filtered. The product was filtered and washed with 2 liters of water and air-dried. The process was repeated about 15 times. After each stage the flameproofness was tested on the air dry material. Up to 4-5 washings there was no loss of flameproofness as shown by lack of afterglow. After about 5 washings the flameproofness started deteriorating as shown by the appearance of atterglow and after about washings the flameproofness was like that of sodium cellulosecarboxylate. A sample of the material laundered 10 times with detergent solution was treated with aluminum sulfate as in Example IV. After washing with water and drying in air, the product showed self-extinguishing properties which were as good as those before laundering.

What is claimed is:

1. A process for flameproofing fibrous cellulosic materials which comprises:

a. the step of oxidizing the cellulosic material to 6- carboxycellulose having a carboxyl content of from about 5 percent to about 25 percent;

b. the step of reacting the 6-carboxycellulose with a compound selected from the group consisting of water-soluble alkali metal hydroxides, buffer solutions of water-soluble alkali metal salts and ammonia until neutralization is effected and a fibrous water-insoluble 6-cellulosecarboxylate is obtained;

0. the step of suspending the product of step (b) in an aqueous solution containing a water-soluble polyvalent cationic salt, the cationic portion of said salt being selected from the group consisting of aluminum, ferric, chromic, zinc, ferrous, cupric, nickel, calcium and magnesium until a water-insoluble, flameproofed, metal 6-cellulosecarboxylate is obtained, the metal being one of those applied in step (c).

2. The process as recited in claim 1 wherein the 6- carboxycellulose has a carboxyl content of at least 8 percent.

3. The process as recited in claim 1 wherein the buffer solution of step (b) is sodium sulfite-sodium bisulfite.

4. The process as recited in claim 1 wherein the water-soluble polyvalent cationic salt of step (c) is aluminum sulfate.

ll 1 l 

2. The process as recited in claim 1 wherein the 6-carboxycellulose has a carboxyl content of at least 8 percent.
 3. The process as recited in claim 1 wherein the buffer solution of step (b) is sodium sulfite-sodium bisulfite.
 4. The process as recited in claim 1 wherein the water-soluble polyvalent cationic salt of step (c) is aluminum sulfate. 